Systemic administration of substance P recovers beta amyloid-induced cognitive deficits in rat: involvement of Kv potassium channels

Abstract

Reduced levels of Substance P (SP), an endogenous neuropeptide endowed with neuroprotective and anti-apoptotic properties, have been found in brain and spinal fluid of Alzheimer's disease (AD) patients. Potassium (K(+)) channel dysfunction is implicated in AD development and the amyloid-β (Aβ)-induced up-regulation of voltage-gated potassium channel subunits could be considered a significant step in Aβ brain toxicity. The aim of this study was to evaluate whether SP could reduce, in vivo, Aβ-induced overexpression of Kv subunits. Rats were intracerebroventricularly infused with amyloid-β 25-35 (Aβ25-35, 20 µg) peptide. SP (50 µg/Kg, i.p.) was daily administered, for 7 days starting from the day of the surgery. Here we demonstrate that the Aβ infused rats showed impairment in cognitive performances in the Morris water maze task 4 weeks after Aβ25-35 infusion and that this impairing effect was prevented by SP administration. Kv1.4, Kv2.1 and Kv4.2 subunit levels were quantified in hippocampus and in cerebral cortex by Western blot analysis and immunofluorescence. Interestingly, SP reduced Kv1.4 levels overexpressed by Aβ, both in hippocampus and cerebral cortex. Our findings provide in vivo evidence for a neuroprotective activity of systemic administration of SP in a rat model of AD and suggest a possible mechanism underlying this effect.

Figure 1. Neuroprotective effects of SP on memory impairments induced by intracerebroventricular injection of Aβ25–35.

(a) Timeline and experimental design. All animals received an infusion (i.c.v.) of Aβ_25–35 (2 µg/µl; 10 µL injection volume) or its vehicle (PBS 10 µL injection volume) and daily treated (7 days) with SP (50 µg/ml/Kg, i.p.) or its vehicle (saline solution 0.9%, i.p.). On the 31^st day after surgery rats were given a daily training session of 4 trials for 3 consecutive days (days 31^st–33^rd). On the 34^th day after surgery the retention of the spatial training was assessed during a 1 min probe trial. On the 35^th day after surgery rats were given a daily training session of 5 trials for 4 consecutive days (days 35^th–38^th). (b) Mean (±S.E.M.) distance traveled to the escape platform on 4 trials of 3 consecutive days of acquisition learning sessions. (c) Time spent (mean ±S.E.M.) during the 1-minute probe trial in the target quadrant and (d) illustrative paths of all animals for the probe test session. (e) Mean (±S.E.M.) distance traveled to the escape platform on 4 trials of 4 consecutive days of the reversal learning sessions (the hidden platform were relocated in a new position each day). * p<0.05 Aβ_25–35/Sal vs PBS/Sal; # p<0.05 Aβ_25–35/Sal vs PBS/SP; $ p<0.05 Aβ_25–35/Sal vs Aβ_25–35/SP. PBS/Sal, n = 10; PBS/SP, n = 10; Aβ_25–35/Sal n = 12; Aβ_25–35/SP, n = 10.

Figure 2. Western blot analysis of Kv 2.1 and Kv4.2 subunit expression in cerebral cortex and hippocampus.

Representative immunoblot of cerebral cortex and hippocampus enriched membrane proteins (50 µg/lane) from (Ctr), Aβ_25–35, Aβ_25–35+SP and SP treated rats. Protein markers are shown at right (in kDa). The immunoreactive signals for a) Kv2.1 and b) Kv4.2 were quantified and normalized against β-actin and expressed as a percentage of control (CTR). Data represent mean (±SEM) from 3 independent experiments. Statistically significant differences were calculated by one-way analysis of variance (ANOVA) for repeated measures followed by Tukey's test for multiple comparisons (**p<0.01 versus Ctr value).

Figure 3. Western blot analysis of Kv1.4 subunit expression in hippocampus and cerebral cortex.

Representative immunoblot of (a) hippocampus and (b) cerebral cortex enriched membrane proteins (50 µg/lane) from (Ctr), Aβ_25–35, Aβ_25–35+SP and SP treated rats. Protein markers are shown at right (in kDa). The immunoreactive signals at 97 and 110 kDa were quantified and normalized against β-actin and expressed as a percentage of the control (Ctr). Data represent mean (±SEM) from 5 independent experiments. Statistically significant differences were calculated by one-way analysis of variance (ANOVA) for repeated measures followed by Tukey's test for multiple comparisons (**p<0.01 versus Ctr value; #p<0.05 versus Aβ_25–35 treatment).

Figure 4. Immunofluorescence analysis of Kv1.4 subunit expression in hippocampus and cerebral cortex.

Upper panel. Representative immunofluorescence photomicrographs showing Kv1.4 expression in a) hippocampus and b) frontal cortex after memory tests in the four experimental treatments: Control (Saline), Aβ_25–35-i.c.v. treated rats (Abeta), Aβ_25–35-i.c.v. and SP-i.p. treated rats (Abeta+SP), SP-i.p. treated rats (SP). Brain sections were labeled with the neuronal marker NeuN (green) and with the anti Kv1.4 antibody (red). As shown by the merge channel all neurons are Kv1.4 positive. Note the diffuse increase in Kv1.4 fluorescence intensity in the Abeta group and the decrease in the Abeta+SP group compared to the Control. Scale bar: a) 20 µm; b) 60 µm. Lower panel. Histograms showing image analysis performed on neuronal cytoplasm (first row) and the surrounding neuropil (second row). The indexes used were: total fluorescence intensity, vesicles diameters, and vesicles fluorescence intensity. Data represent means (±S.E.M.) obtained from three independent experiments. Statistically significant differences were calculated by one-way analysis of variance (ANOVA) for repeated measures followed by Tukey's test for multiple comparisons (**p<0.01 versus Saline; #p<0.05, ##p<0.01 versus Aβ_25–35treatment).

Figure 5. SP reduced Aβ25–35-induced overexpression of Kv1.4 subunit in rat hippocampal neurons.

a) Example of Western blot obtained from hippocampal cultures exposed to 20 µM Aβ_25–35 (Aβ alone or in the presence of SP (100 nM) and analyzed 48 h later using a polyclonal antibody against Kv1.4 subunit. The same blots were stripped and reprobed with an antibody against β-actin as internal control (lower panels). Quantitative analysis is depicted below the blots and was determined by band densitometry analysis considering the values found in CTR cells as 100. Data represent means (±S.E.M.) obtained from 4 independent experiments run in duplicate. (**p<0.001 versus CTR, #p<0.05 versus Aβ_25–35 treatment). b) Representative immunofluorescence photomicrographs showing Kv1.4 expression in primary hippocampal cultures. Note the increase in immunofluorescence in the Aβ_25–35 neurons, as compared to control neurons, reversed by SP treatment. Images were obtained from three independent experiments. Scale bar: 20 µm.